Tryit Editor v1.4

<html>
<head>
<script type="text/javascript">
function selectMultiple()
{
document.getElementById("mySelect").multiple=true;
}
</script>
</head>
<body>

<form>
<select id="mySelect" size="4">
	<option>Apple</option>
	<option>Pear</option>
	<option>Banana</option>
	<option>Orange</option>
</select>
<input type="button" onclick="selectMultiple()" value="Select multiple">
</form>
<p>Before you click on the "Select multiple" button, try to select more than one option (by holding down the Shift or Ctrl key). Then click on the "Select multiple" button and try again.</p>

</body>
</html>

Your Result:

Edit the code above and click to see the result.

W3Schools.com - Try it yourself